Method for measuring density of a bulk material in a stockpile

ABSTRACT

A method for determining the density of coal or other bulk material in a stockpile includes boring a calibration hole into the pile and collecting at least a portion of the cuttings thus obtained into a container. A series of readings are taken within the container with a nuclear depth-density gauge, and the weight of the cuttings and container volume is measured. The measured weight and volume are compared with the gauge readings to calibrate the gauge. A test hole is then bored substantially to the bottom of the stockpile, and a section of steel casing is inserted and partially pressed into the hole. A second section of casing is attached to the first and similarly pressed into the hole. In this manner, casing is extended the full depth of the hole. Readings are then taken at various depths therein with the nuclear gauge. From the data thus obtained, average density throughout the pile is determined.

.Iadd.This is a continuation of Ser. No. 131,293 filed Dec. 9, 1987, nowabandoned, which is a continuation of Ser. No. 853,280 filed Apr. 17,1986, now abandoned. .Iaddend.

BACKGROUND OF THE INVENTION

The present invention relates to a method for determining the density ofa bulk material at a plurality of points within a stockpile, and moreparticularly, to such a method that utilizes a nuclear depth densitygauge for determining the density of the material at the various pointsthroughout the pile.

Electrical generating companies and other businesses frequently keep ininventory large quantities of coal, stored in outdoor stockpiles. Forbusiness planning, financial reporting, regulatory and other reasons, itis often necessary to determine relatively accurately the quantity ofcoal contained within a stockpile. The quantity is customarily expressedin terms of its weight, but it is clearly impractical to determine thequantity by physically weighing the coal. Thus, the typical methodrelies instead upon determining the density of the coal and the volumeoccupied by the stockpile, from which the weight of the coal ininventory may be calculated.

Several methods are known for determining the density of coal within astockpile. Since the density is not necessarily uniform throughout thepile, any method must rely upon a plurality of density measurementstaken at various locations throughout the pile. In one method, known asthe volumetric displacement method, a cylindrical rubber bag filled withwater is used to determine the content of holes augered in a number oflocations throughout the pile. The coal cuttings from each of the holesare collected and weighed. The volume of the hole from which the coalcuttings are removed is determined by placing the bag within the holeand filling the bag with water. The volume of water to fill the hole isrecorded, and from the volume and weight of the cuttings, the density ofthe coal at that location may be calculated.

The displacement method presents a number of disadvantages. Measurementscan be made only relatively near the top surface of the pile, and nodensity variation with increasing depth can be detected. The coalcuttings must be very carefully collected during augering, since anyloss would affect the accuracy of measurement. The method is generallytime-consuming and awkward to perform since, for example, the waterplaced into the rubber bag must be pumped back out prior to removal ofthe bag for subsequent measurements.

A second method for measuring density utilizes a nuclear depth densitygauge for measuring density at a number of points in the pile. The gaugeincludes a probe consisting primarily of a source of radiation and asensing element or detector. The detector is connected by a cable to arecording instrument or scaler. The density measurement is performed bylowering the probe through access tubing placed within a hole to thedesired depth. The probe effectively measures the density of a generallyspherically shaped volume approximately 5 inches in radius.

The detector within the gauge probe receives gamma radiation, the amountof which is recorded by the scaler. The probe source is a radioactivematerial that emits such radiation at a constant average rate. The gammarays interact in various processes at the atomic level with thesurrounding medium. The number of interactions, or scattering events,per unit time is a function of the density of the medium. Thedetermination of sufficient quantities of the back scattered radiationwithin a certain fixed energy range and on a per unit time basis willgive a statistically significant measure of the relative degree ofscatter by materials of different densities.

One problem in using the nuclear depth density gauge is providing forinsertion of the probe into the stockpile. A hole must be formed, and anaccess tubing inserted therein, into which the probe is placed. Theprobe is designed for use with 1.9 inch inner diameter, 2.0 inch outerdiameter aluminum tubing, and the gauge is designed primarily formeasurements in soil. In such a case, augering of a hole and insertionof the tubing is a relatively simple matter. In coal stockpiles,however, necessary measurement depths can be as great as 100 feet ormore. The access tubing must fit snugly within the hole to achieveaccurate results, but at such depths the aluminum access tubing does notpossess sufficient strength to withstand its insertion the full lengthof the hole.

In one known method, a hollow-stem auger is used to advance the hole toa point above where the gauge reading is to be taken. The auger isdisconnected from the drill rig but is left in place within the coal. Alength of steel casing, having a split-spoon sampler attached to itslower end, is inserted into the hollow portion of the auger. The samplerand casing are then driven approximately 1 foot into the coalimmediately beneath the auger, and the coal contained within the sampleris removed. The sampler is then replaced by a length of aluminum tubingat the end of the steel casing, which is then reinserted into the hole.The aluminum section is forced into the portion of the hole formed bythe sampler, and a density measurement is taken therein. The casing isthen removed, the auger is reattached to the drill rig, and the hole isadvanced to just above the location of the next measurement.

This method represents a relatively complex and time-consumingprocedure, particularly since a number of measurements must be taken atvarious depths along each of a number of holes in the stockpile. Sincethe accuracy of the density determination improves as the number ofmeasurements is increased, it can easily be seen that lack of a simplemethod for installing access tubing along the full length of the holerepresents a significant disadvantage.

A second problem associated with nuclear depth density gaugemeasurements in coal stockpiles results from the fact that suchinstruments are designed with the expectation that they will be usedprimarily in soil. In preparing the instrument for any use, acalibration must be determined to convert the back scattering radiationcount received by the scaler into a corresponding density value. Suchcalibration is performed by the instrument manufacturer, but isperformed such that the instrument is properly calibrated for use intypical soils. While such a calibration is sufficiently accurate forusing the gauge at construction sites and the like, coal is sufficientlydifferent material that the factory calibration values are not usable.

One method for recalibrating the gauge relies upon the difference inchemical make-up between coal and soil. The mathematical formula used inproducing the calibration curve includes a constant factor which isrelated to the chemical composition of the material to be tested. Thus,using coal of a known density, it is possible to calculate a newconstant which may then be inserted into the formula.

One disadvantage to this method, however, is that the chemicalcomposition and relative proportions of coal from one stockpile toanother is not constant, but rather various widely depending upon wherethe coal was mined. Thus, for the calibration to be accurate, it must berepeated prior to density measurement in each stockpile to be tested.Since the determination of the constant must be performed undercarefully controlled conditions such that all other formula factorsremain constant, this becomes a time consuming, tedious procedure.

The foregoing is equally applicable to density measurements of any otherbulk material stored within a stockpile.

Accordingly, what is needed is a new method for determining the densityof bulk material, particularly coal, stored within a stockpile. Such amethod should utilize the nuclear depth density gauge due to its easeand simplicity of operation, but should reduce to a minimum the awkwarddrilling technique using the hollow-stem auger. Calibration should berelatively easy to perform, and capable of performance prior tomeasurement in each stockpile.

SUMMARY OF THE INVENTION

The present invention provides a method for determining the density of abulk material such as coal in a stockpile. A calibration hole is boredinto the pile, and at least a portion, but not necessarily all, of thecoal cuttings obtained therefrom is collected into a container of apredetermined tare weight and volume. A length of steel casing isdisposed within the container, extending the height of the container andremote from the walls thereof. As the container is filled, the coal iscompacted to approximately a first density value. The inside of thelength of casing is kept free from coal.

The probe of a nuclear depth density gauge is inserted into the lengthof casing and a series of timed readings are taken with the gauge. Theprobe is moved along the length of casing between each reading, suchthat each reading is taken at a location along the length of casingseparated by a predetermined interval from the last preceding reading.Thus, each reading taken corresponds to one of a plurality ofpredetermined locations along the casing. The container and the coalcuttings held therein are then weighed.

A second container of predetermined tare weight and volume is filled, orthe first container is refilled, with coal cuttings obtained from thecalibration hole. During filling, the coal is compacted to approximatelya second, different density. The gauge probe is inserted into the lengthof casing within the container, and a series of timed readings are takenat a plurality of locations along the casing. The second container andthe coal cuttings therein are then weighed.

Similarly, a third container compacted to approximately a third densityis prepared, has a plurality of gauge readings taken along the length ofcasing therein, and is weighed.

The density of the coal cuttings within the container or containers iscalculated based on the volume, tare weight and loaded weight of thecontainers. The calculated densities of the coal are then compared withreadings obtained with the gauge, whereby a calibration curve may bedeveloped for the coal stockpile.

Following calibration of the nuclear depth density gauge, a test hole isbored substantially to the bottom of the stockpile. A first section ofsteel casing of an outer diameter substantially equal to the diameter ofthe test hole is inserted into the hole. A pressing force is applied toone end of the casing section until the section is pushed partially intothe hole. A second section of casing is connected to the upper end ofthe first section, and a pressing force is applied to the upper end ofthe second section until the second section is pressed partially intothe test hole, whereby the first section is pushed further into the testhole. Additional sections of casing are connected and pressed into thehole until the casing sections extend the full depth of the hole.

The probe of the nuclear depth density gauge is inserted into the casingsections and located at a predetermined depth along the sections. Aseries of time readings are taken with the gauge. The probe is movedalong the casing sections such that at the beginning of each of thereadings, the probe is at a location along the test hole separated by apredetermined interval from its location at the last preceding one ofthe readings. Thus, each of the readings taken corresponds to one of aplurality of predetermined depths along the test hole.

The readings thus obtained are compared with the calibration curvedeveloped for the pile, whereby a density value for each test reading isobtained.

Accordingly, it is an object of the present invention to provide amethod for determining the density of bulk material in a stockpileutilizing a nuclear depth density gauge; to provide such a method thatincludes a relatively simple and direct means for insertion of accesstubing with which the gauge is used into the stockpile; to provide sucha method that is usable at any depth within the pile; to provide such amethod that includes relatively simple means for calibration of thegauge for use in the material, and to provide such a method in which thecalibration method is sufficiently simple to enable its use prior tomeasurements within each of a number of stockpiles. .[.p Other objectsand advantages of the present invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.].

.Iadd.Other objects and advantages of the present invention will beapparent from the following description, the accompanying drawings andthe appended claims.Iaddend..

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cut-away view of a coal stockpile showing a truck-mounteddrill ring as used in the method of the present invention;

FIG. 2 is a perspective view of the rear portion of a pick-up truckshowing apparatus for weighing loaded calibration barrels;

FIG. 3 is a cut-away view of a loaded calibration barrel showing anuclear depth-density gauge mounted for the taking of calibrationreadings;

FIG. 4 is a cut-away view of a portion of a stockpile showing the casinginserted thereinto and the nuclear depth-density gauge mounted for thetaking of test readings;

FIG. 5 is a side elevational view showing a nuclear surface densitygauge as used in the method of the present invention; and

FIG. 6 is an example of a typical calibration curve generated as part ofthe method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of the present invention for determining the density of abulk material at a plurality of locations within a stockpile makes useof a nuclear depth density gauge for density readings. The method, whichis particularly adapted for use in coal, is also usable with a number ofother materials such as bauxite, salt, and the like, or other bulkmaterials capable of supporting a boring, such materials being generallyreferred to as "cohesionless". Materials such as grain or dry sand,within which a boring will have a strong cave-in tendency, are generallynot suitable, nor are materials such as large pieces of scrap metalwhich have large amounts of open space between material particles. Thus,while the present invention is discussed with reference to use with acoal stockpile, it will be understood that the invention encompasses thepractice of the method with other bulk materials as well.

In the preferred embodiment, the nuclear gauge is a Troxler model No.1351 nuclear depth gauge with an 8 millicurie cesium-137 source, whichis coupled to a Troxler model No. 600 scaler. Although the invention isnot limited to these particular devices, the two preferred devices aremanufactured by Troxler Electronic Laboratories, Inc. of ResearchTriangle Park, N.C.

The bulk of the density determination method is performed in the fieldat the stockpile site, and consists essentially of two stages. Thenuclear gauge utilized for density determination does not provide asignal that can be displayed directly as a density measurement on thescaler. The reading obtained represents a measurement of the number ofnuclear back scattering events recorded by the gauge sensor, and thisnumber of events must then be converted into a meaningful densitymeasurement. Normally, the gauge is provided with various graphs and/orcharts prepared at the factory for the purpose of converting the countnumber to a density measure. Since the relationship between counts anddensity is dependent upon the atomic structure of the material beingtested, however, these factory calibrations are prepared for maximumaccuracy within a broad range of materials that it is expected the gaugewill be used in. Such gauges are typically used in soils, and since theatomic structure of coal is significantly different from soils, thefactory calibrations are inappropriate for use for densitydeterminations in coal or other materials.

Thus, the first stage of the density determination of a stockpile is theproduction of satisfactory calibration curves for use in coal.

As seen in FIG. 1, a truck 10 having a conventional boring rig 12mounted thereon is driven onto the coal stockpile 14. A number ofborings 16 are made at various locations on the stockpile 14 using, forexample, 6 inch O.D. continuous flight augers. These augers, as well asaugers used later within the method, may be any appropriate commericallyavailable augers, such as those available from McLaughlin Mfg. Co. ofPlainfield, Ill. These borings 16, which typically will number at leasttwo located randomly along the stockpile 14, are made to the bottom ofstockpile 14, and the coal cuttings resulting from the borings arecollected.

Enough cuttings are collected to fill a calibration barrel 18, shown inFIG. 2. Three calibration barrels are prepared, with one barrel 18 usedthree times, or three such barrels 18 used once each. Barrel 18 ispreferably constructed from heavy gauge steel, and is open at the top. Alength of steel casing 20, preferably of 1.90625 (1-29/32) inch innerdiameter and 2.25 (21/4) inch outer diameter is permanently mountedwithin barrel 18 so as to extend vertically from the base of barrel 18to slightly above its upper end. Casing 20 is further mounted so as tobe exactly centered within barrel 18.

Each barrel 18 preferably has a volumetric capacity of approximately 8cubic feet, although prior to initial use, the exact volume must bedetermined and recorded. Additionally, the empty or tare weight ofbarrel 18 must be determined and recorded.

The coal cuttings are placed into each of the calibration barrels 18 toachieve three different degress of compaction for three differentdensities. The approximate values for each density are preselected, withthe lowest density approximating the lowest density value expected to beencountered within the stockpile, the highest value approximating thehighest expected value, and a third value that is intermediate of thesevalues. Care must be taken while filling the barrels to avoid gettingany coal inside the casing 20.

The coal cuttings are placed fairly loosely into the first of thecalibration barrels 18. The coal must be carefully placed within thebarrel 18, however, so as to achieve relatively uniform densitythroughout the barrel. The barrel 18 is filled to its brim and levelledoff. In one example, this will result in approximately 425 pounds ofcoal within the barrel, to result in a density of approximately 55pounds per cubic foot.

Following the loading of barrel 18, the nuclear depth density gauge isused to take a series of density readings within barrel 18. As seen inFIG.3, the nuclear gauge 50 includes a shield 52 and a probe 54. Probe54 contains the radioactive source and the back scattering detector,along with a small preamplifier for amplifying the counts received bythe detector. A cable 56 is connected to probe 54, for transmitting theelectrical signals corresponding to counts recorded by the detector.Cable 56 passes through shield 52, which is constructed to be mountableat the upper end of a length of approximately sized tubular casing, suchas casing 20. A simple cable clamp 58 is provided for cable 56 at theupper end of shield 52, so that probe 54 may be suspended at a desireddistance beneath shield 52. Additionally, probe 54 is retractable withinshield 52, the walls of which are constructed to provide effectiveradioactive shielding. Shield 52 further includes a pair of handles 60to facilitate movement thereof.

Cable 56 connects probe 54 with a scaler 62 having an appropriatedigital readout 64. Scaler 62 serves to accumulate the counts collectedby the sensor of probe 54, and displays the total number of counts sorecorded on display 64. Scaler 62 includes appropriate controls (notshown) and an internal clock means, so that counts may be recorded anddisplayed to correspond to accumulations thereof over a predeterminedselected time period.

The nuclear gauge 50 is turned on and permitted to stabilize. Sinceshield 52 includes radioactive shielding, the interior of shield 52serves as a controlled environment. The probe 54, which can be drawninto shield 52 when the gauge 50 is not is use, is left within shield 52for the taking of several standard counts to insure proper operation ofgauge 50 prior to use. A series of such counts, for example, 10 countsof a duration of 1-4 minutes each, is taken and recorded.

Following the taking of the standard counts, the nuclear gauge probe 54is removed from the interior of shield 52 and inserted into casing 20within calibration barrel 18. Shield 52 is then placed on top of casing20, as shown in FIG. 3. Probe 54 is lowered to the bottom of casing 20,and then is raised two inches therefrom. Since the radioactive source iscontained within probe 54 at its lower end, the effective site of areading taken with probe 54 is at its lower end.

A reading for a predetermined time period is taken with probe 54positioned two inches above the bottom of barrel 18. The count isrecorded, the probe is raised an additional two inches, and a secondreading is taken. In similar fashion, readings are taken and recorded attwo inch intervals along casing 20.

The loaded calibration barrel 18 is then weighed. Referring back to FIG.2, the preferred apparatus for weighing the loaded barrels 18 may beseen. An overhead beam 22 is supported by two pairs of legs 24 (only onepair shown) that are in turn mounted to the bed of a conventionalpick-up truck 26. An electrically driven winch 28 is mounted to thedistal end of beam 22, and includes the necessary wiring 30 extending toan appropriate control (not shown). A conventional spring scale 32,having a preferred capacity of 1,000 pounds, is suspended from hook 34attached to the winch cable 36.

A pair of rings 38 are permanently attached opposite each other near thetop of the exterior of barrel 18. A length of chain 40 is connectedbetween rings 38. Winch 28 is operated to lower hook 34 so that the hook42 of attached scale 32 may be engaged with chain 40. Winch 28 is thenreversed, so as to lift barrel 18 from the ground through scale 32.

The weight of the loaded barrel 18 is then recorded from scale 32. Bysubtracting the tare weight of barrel 18, the net weight of the coalcontained within barrel 18 is obtained. The barrel 18 is then loweredand disconnected from scale 32.

Following weighing, calibration barrel 18 is either emptied or a secondcalibration barrel 18 is used. The barrel 18 is filled with the coalcuttings, which are uniformly tamped during placement within barrel 18.The coal is compacted within the barrel to a second approximate density.For example, approximately 500 pounds of coal may be placed therein, foran approximate density of 65 pounds per cubic foot. The nuclear gauge 50is then used to take a series of readings along casing 20 of the secondbarrel, in a manner identical to the taking of the readings within thefirst barrel. The barrel 18 is then weighed using the apparatus shown inFIG. 2.

Once it has been weighed, the calibration barrel 18 is either emptied ora third calibration barrel is used. The coal is placed within the barreland uniformly tamped to a greater degree than the second barrel so that,for example, approximately 580 pounds of coal are placed within thebarrel. An approximate density of 75 pounds per cubic foot is thusproduced. During the filling of the third calibration barrel, it may benecessary to spray water uniformly throughout the coal within the barrelto achieve the desired compaction.

A series of readings are taken within the third calibration barrel 18using the nuclear gauge and the barrel is then weighed, in a manneridentical to that used with the first two barrels.

Sufficient data has now been acquired to enable calibration of thenuclear gauge for the actual density measurements. Since the calibrationcalculations need not be made in order for the measurements to be taken,however, the data will typically be recorded and then analyzed at alater time or relayed to a remote location for immediate analysis. Thisminimizes the actual time that must be spent at the stockpile site.

Details of the calibration calculations will be discussed in detailbelow. Once the calibration readings have been taken with the nucleargauge 50 for the particular coal within the stockpile 14, the secondstage of the method for density determination, the taking of densityreadings within the coal pile, may be begun. It should be recognized,however, that if desired, the taken of readings within the stockpile maybe performed prior to the taking of calibration data. The first of aseries of test bores are made within the stockpile 14, each bore similarto bore 16 shown in FIG. 1, and extending to the bottom of pile 14. Eachtest bore is made with a 2.25 inch outer diameter continuous flightauger. After the auger is removed from the bore, a section of 2.25 inchouter diameter, 1.90625 inch inner diameter steel casing is insertedinto the bore and pushed thereinto until all but the uppermost portionof the section is contained within the bore. While the given dimensionsrepresent preferred values for augers and casing, it should berecognized that bore diameter and casing outer diameters should besubstantially equal so that the outer casing surface and the coal aresubstantially in contact.

A second section of casing is then attached to the first, and is pushedinto the bore until all but its uppermost end is contained within thebore. In similar fashion, sections of casing are connected and insertedinto the bore until the casing extends the full length of the bore tothe bottom of the stockpile 14. The sections of casing used within thebore are preferably of a commercially available type generally referredto as BXWL, available for example from Christensen Dia-Min Tools, Inc.The casing is typically available in sections of 5-10 feet. Theadvantage of this particular type of casing is that the threads forjoining the sections are provided such that both the inner and the outersurfaces of the casing are essentially smooth and continuous, even atthe section joints.

It is important to note that the casing utilized within the presentmethod is formed from steel, while the nuclear gauge is intended for usewith aluminum casing. Moreover, nuclear gauges of the general typeutilized in the present invention are typically supplied (and factorycalibrated) with a 2 to 3 millicurie source, whereas an 8 millicuriesource is used for the present invention to obtain necessary radiationpenetration through the steel casing. Nonetheless, it will be recognizedthat since the nuclear gauge is recalibrated prior to use within theparticular stockpile, and since the calibration is made within a lengthof steel casing, the change in casing material and the source isaccounted for within the calibration procedure.

It is additionally important to note that in inserting the casing intothe bore, the casing is generally pushed with a continuous force as itis inserted. This is easiest accomplished by utilizing the hydraulicauger advance mechanism of the drilling rig, which supplies sufficientpushing force to insert the casing into the bore. The casing may also beinserted by other force appliation means and is not necessarily limitedto pushing forces. For example, a vibration driver may be adapted toinsert the casing into the hole. Driving the casing by a series of blowsshould be avoided, however, since this will disturb the coal surroundingthe casing, possibly resulting in an inaccurate density measurement. Ithas been found that limited driving by blows may be necessary in someinstances to insert the casing, but such driving should be used only asneeded.

It should be recognized that any required driving of the casing by blowsis made possible only through use of the steel casing. Driving of thealuminum casing typically used with nuclear gauges, or even insertioninto the stockpile of such casing by application of a continuous pushingforce, will tend to bind, deform or otherwise damage the casing.

To further facilitate the insertion of casing into the bore, a cone tip(not shown) is threadingly connected to the lower end of the lowermostlength of casing. This tip also serves to prevent coal from entering thecasing during insertion.

Once the casing is positioned within the bore, the ptobe 54 of thenuclear gauge 50 is inserted into the casing and lowered to the bottomthereof. The gauge shield 52 is positioned on the top of casing 70 asseen in FIG. 4. A density reading of the same time period as thecalibration counts is taken and recorded. The probe 54 is then raised apredetermined interval along the casing 70, and a second reading istaken and recorded. In similar fashion, readings are taken atpredetermined intervals along the entire length of casing 70. Thereadings thus obtained are then recorded for later conversion to densityvalues corresponding to the locations within the stockpile 14 at whichthe readings were taken.

As will be explained in greater detail below, a number of bores areaugered at a plurality of locations on the stockpile 14, with a seriesof readings taken with the gauge 50 along the length of each bore. Theprocedure for each bore will, of course, be identical to that describedabove.

Although the nuclear depth density gauge 50 is highly accurate formeasurements within the stockpile 14, its accuracy is significantlyreduced with the top 1 or 2 feet of coal within the pile. Typically,however, this region of the pile frequently represents an area wherevariations with density relative to the remainder of the pile may beexpected to be found. Thus, determination of density at a plurality ofpoints within this region is highly desirable.

Coal density within this region is therefore determined through use of anuclear surface density gauge, with the preferred instrument being aTroxler model No. 3401-B surface moisture-density gauge, manufactured byTroxler Electronic Laboratories, Inc.

As seen in FIG. 5, the surface gauge 80 includes a radioactivelyshielded housing 82 through which a probe 84 is vertically movable. Aradioactive source 86, preferably an 8 millicurie cesium-137 source, ismounted near the lower end of probe 84. An index rod 88 is fixedlymounted to housing 82, and a squeeze-type clamp 90 is operablypositioned on rod 88. Clamp 90 is attached to the upper end of probe 84,so that probe 84 may be raised or lowered by clamp 90, and once probe 84is in a desired position, clamp 90 may be used to secure the rod thereinby securing clamp 90 to index rod 88.

One or more gamma ray detectors 92 are mounted to gauge housing 82 forreceiving gamma radiation from source 86 following passage of theradiation through the material to be tested along, for example, lines93. A control portion 94 having an appropriate control panel and digitalread-out (not shown) is also provided.

As with the depth density gauge 50, the operation of the surface gauge80 is dependent upon interaction at the atomic level of gamma radiationfrom source 86 with the atoms of the material being tested. A certainportion of the radiation will be scattered such that it will be receivedby the detector 92, with the amount of radiation received beinginversely proportional to the density of the material. Provision is madewithin the controls of the gauge 80 for the taking of accurately timedreadings over a preselected interval, which are then readable from thedigital display.

For the same reasons as with the depth density gauge, the surface gauge80 must be calibrated prior to use on any individual stockpile. Thegauge 80 is turned on and permitted to stabilize, following which aseries of standard counts is taken to insure proper gauge operation. Thesource end of probe 84, which is carried within shielded housing 82 whengauge 80 is not in use, is left within housing 82 for the standardcount.

For use of gauge 80 within the material to be tested, a metal rod (notshown) of a diameter identical to probe 84 is provided. The rod isdriven with a hammer or other means into the material to a point justbelow the maximum depth at which the gauge 80 is to be used. The rod isremoved, and the gauge 80 is positioned on the surface such that probe84 can be inserted into the hole formed by the rod. For accurateresults, it is necessary that the gauge housing 82 be in contact withthe material surface, and thus prior to measurement, it may be necessaryto level the surface.

The calibration procedure for the surface gauge 80 is generally similarto that used for the depth density gauge 50. A series of threecalibration barrels are prepared, which may be performed using threebarrels or by using a single barrel three times. The barrel is generallysimilar in appearance to that shown in FIG. 2, but does not include acentrally mounted length of casing. Additionally, since the depth atwhich the surface gauge is used is less than that for the depth gauge,it is preferred that a smaller calibration barrel to be provided so asto reduce the amount of coal required and the time needed for fillingand compacting the coal within the barrel. Thus, a barrel having apreferred volume of approximately five cubic feet is used. The coal usedfor the calibration is collected randomly from a preferred minimum ofsix locations on the surface of the stockpile until sufficient coal isaccumulated to fill the calibration barrel. As with the depth gauge,each barrel is compacted to a different approximate densitycorresponding to the expected high and low density values, plus oneintermediate value. In one example, about 300 pounds of coal is placedinto the first barrel, for an approximate density of 60 pounds per cubicfoot. After filling, a series of readings are taken within the coalwithin the barrel by gauge 80. A hole is made in the coal near thecenter of the barrel, and the gauge 80 is positioned and the probe 84inserted into the hole. Three one-minute test counts are made with thegauge in this position. A second hole is then made in such a locationthat the gauge can be rotated 90° on the coal surface and another set ofthree readings is taken and recorded. The barrel is then weighed withthe apparatus shown in FIG. 2.

A second barrel is then prepared with, for example, about 350 pounds ofcoal for an approximate density of 70 pounds per cubic foot. The readingand weighing procedure is repeated for this barrel, and then a thirdbarrel is prepared with, for example, about 400 pounds of coal,compacted to approximately 80 pounds per cubic foot. The procedure isthen .[.repreated.]. .Iadd.repeated .Iaddend.for this third barrel.

Once the calibration data has been taken with the gauge 80, a series ofreadings is taken along the surface of the stockpile. (This sequencemay, of course be reversed.) The data is then recorded for lateranalysis.

Once all the data has been collected at the stockpile site in the mannerdescribed above, it may be analyzed to determine the coal density withinthe stockpile and, hence, the quantity of coal contained therein. Thecalculations will be described as performed with the data collected bythe depth gauge 50. It should be understood, however, that thecalculation regarding the data obtained with the surface gauge 80, whileperformed independent from those pertaining to the depth gauge, areaccomplished in an identical manner.

The average calibration count taken within each of the three calibrationbarrels is determined first. This value represents the average of thecalibration count readings taken at the two inch intervals throughoutthe depth of each calibration barrel.

The actual density of the coal in pounds per cubic foot for eachcalibration barrel is determined by dividing the measured weight of thecoal in the barrel by the volume of the barrel. The calibration countaverage value and actual density for each of the calibration barrels arethen used to develop the calibration curve for the depth gauge 50.

The calibration curve is developed from the mathematical relationshiplinking the count values with the actual coal density at the point wherethe count value is obtained. While the actual relationship is relativelycomplex, the relationship may be approximated over the range of densityvalues typically encountered in coal stockpiles by the expression

    N=Ae.sup.B ρ,

where N is the measured count value, A and B are constants, and ρ is theactual density at the point in question. It should be readily seen thatthis equation may be rewritten as

    ln N=Bρ+ln A,

which represents the slope-intercept form of the equation for a straightline. The three data points generated as a result of the calibrationmeasurements are then used by a computer to determine the slope andintercept of the straight line using the "least squares fit" method. Thestraight line, which may be preferably plotted on a semi-logrithmicgraph for ease of use, as seen in FIG. 6, is used as the calibrationcurve for the data points developed within the stockpile.

The actual density values may be determined either graphically byreference to a plot such as shown in FIG. 6, or the data may be fed intothe computer for calculation using the equation set forth above. Due tothe enhanced speed and accuracy obtained by using the computer, thislatter method is preferred.

In an identical fashion, the data collected with the surface gauge isconverted to density value.

Prior to the collection of data at the stockpile site, it is necessaryto determine the number of data points to be taken and, hence, thenumber of bores to be made into the stockpile. While the actual decisionis in large part based upon the needs and desires of the particularstockpile owner, the preferred method for determining the necessarynumber of data points utilizes a statistical approach that takes intoconsideration the variation in density values normally expected to beobserved within a stockpile.

The preferred statistical approach is based upon the relationship thatthe mean of a sample population reflects the central tendency of theactual population, if the sample size is of a sufficient magnitude. Thesample size is determined by using the following equation

    n=[(2Z.sub.α/2 σ)/E].sup.2,

where n is the sample size, Z.sub.α/2 is the effect of the confidencelevel for a normal distribution E is the confidence width, and σ is thestandard deviation of the population distribution.

Provided that the population is normally distributed, the mean of thesample values approximates the mean value of the population. Thus,Z.sub.α/2 is taken from standard normal curves for the desiredconfidence level. In making the present analysis, the confidence levelwas selected as 98%, for which Z.sub.α/2 is 2.33. The confidence widthwas selected as ±2 pounds per cubic foot, and the standard deviation wastaken from historical records for typical coal piles.

After the sample size is determined, the number of borings is determinedtaking into account the desired sample separation and the approximatedepth of the proposed borings. A random number system is then used tolocate the borings on the pile.

Once the density test values are obtained, they are averaged and astandard deviation of the sample population is determined. The newlycalculated standard deviation and the desired confidence level and widthare used to ascertain whether the actual sample size is sufficient. Ifthe sample size is sufficient, then the mean density determined by thetesting procedure represents the mean density of the coal stockpilewithin the limits of the control .[.therion.]. .Iadd.thereon.Iaddend..

At some point before, during, or after the density determination withthe stockpile has been made, the volume occupied by the pile isdetermined through a conventional, known method, such as aerialphotography. The volume thus obtained is multiplied by the main densityvalue, yielding the total quantity by weight of coal or other materialcontained within the stockpile.

While the methods herein described constitute preferred embodiments ofthis invention, it is to be understood that the invention is not limitedto these precise methods, and that changes may be made therein withoutdeparting from the scope of the invention which is defined in theappended claims.

What is claimed is:
 1. A method of determing the density of cohesionlessbulk material in a stockpile, comprising the steps of:completely boringa test hole substantially to the bottom of said stockpile; followingcomplete boring of said hole, positioning one end of a first section ofsteel casing of an outer diameter substantially equal to the diameter ofsaid test hole at or near the opening to said hole; applying a firstforce to the opposite end of said first section until said section ispushed partially into said hole; connecting a second section of casingto the upper end of said first section; applying a force substantiallyequivalent to said first applied force to the upper end of said secondsection until said second section is pressed partially into said testhole, whereby said first section is driven further into said test hole;connecting additional sections of casing and inserting said sectionsinto said test hole until said casing sections extend the full depth ofsaid test hole; after said casing sections extend the full depth of saidhole, inserting the probe of a nuclear depth-density gauge into saidtest hole and locating said probe at a predetermined depth along saidhole, said probe having a length of cable attached thereto; taking aseries of timed readings with said gauge; and moving said probe alongsaid test hole such that at the beginning of each of said readingsfollowing the first thereof, said probe is at a location along said testhole separated by a predetermined interval from its location at thebeginning of the last preceding one of said readings, whereby each ofsaid readings taken corresponds to one of a plurality of predetermineddepths along said test hole.
 2. A method as defined in claim 1, whereinboring of said test hole is performed with a truck-mounted,hydraulically advanced drill rig.
 3. A method as defined in claim 1,wherein application of said forces are at least partially performed byapplication of a substantially continuous pressing force.
 4. A method asdefined in claim 3, wherein boring of said test hole is performed with atruck-mounted, hydraulically advanced drill rig.
 5. A method as definedin claim 4, wherein said continuous pressing force is applied with thehydraulic advance mechanism of said drill rig.
 6. A method as defined inclaim 1, wherein said first, said second, and said additional sectionsof casing are BXWL, said sections having threads for joining saidsections formed thereon such that both the inner and the outer surfacesof said casing are essentially smooth and continuous, including jointstherealong at which said sections are joined.
 7. A method as defined inclaim 1, wherein during the taking of each of said readings, said probeis held in a substantially stationary position within said hole,movement of said probe being performed between at least some of saidreadings.
 8. A method as defined in claim 1, comprising the furthersteps of:boring a plurality of calibration holes into said stockpile ata plurality of spaced-apart locations thereon; collecting at least aportion of the cuttings obtained from said boring into a container ofpredetermined tare weight and volume having at least one wall, saidcontainer having a length of steel casing disposed therein extending theheight of said container remote from the wall thereof, said length ofcasing being kept free from said cuttings; inserting said probe of saidnuclear depth-density gauge into said length of casing and locating saidprobe at a predetermined position along said length of casing; taking aseries of timed readings with said gauge; moving said probe along saidlength of casing such that at the beginning of each of said readingsfollowing the first thereof, said probe is at a location along saidlength of casing separated by a predetermined interval from its locationat the beginning of the last preceding one of said readings, wherebyeach of said readings taken corresponds to one of a plurality ofpredetermined locations along said length of casing; weighing saidcontainer and said cuttings held therein; calculating the density ofsaid cuttings within said container based on the volume, tare weight andloaded weight of said container; and comparing the calculated density ofsaid cuttings with said readings obtained with said gauge, whereby saidguage may be calibrated for the material in said stockpile.
 9. A methodas defined in claim 8, comprising the further step of prior to insertingsaid probe into said casing within said container, compacting saidcuttings within said container to provide material therein atapproximately a predetermined density.
 10. A method as defined in claim8, wherein the bulk material within said stockpile is coal. .Iadd.
 11. Amethod of determining the density of a cohesionless bulk material in astockpile, comprising the steps of:completely boring a test holesubstantially to the bottom of said stockpile; following complete boringof said hole, positioning one end of a first section of casing of anouter diameter substantially equal to the diameter of said test hole ator near the opening to said hole, said casing being formed of a materialand of a thickness to provide said casing with sufficient strength towithstand insertion into said hole; inserting said first sectionpartially into said hole; connecting a second section of casing to theupper end of said first section; inserting said second section partiallyinto said test hole, whereby said first section is inserted further intosaid test hole; connecting additional sections of casing and insertingsaid sections into said test hole until said casing sections extend thefull depth of said test hole; inserting the probe of a nucleardepth-density gauge into said test hole and locating said probe at apredetermined depth along said hole, said probe having a length of cableattached thereto; taking a series of timed readings with said gauge; andmoving said probe along said test hole such that at the beginning ofeach of said readings following the first thereof, said probe is at alocation along said test hole separated by a predetermined interval fromits location at the beginning of the last preceding one of saidreadings, whereby each of said readings taken corresponds to one of aplurality of predetermined depths along said test hole. .Iaddend.